SAN FRANCISCO (AP) — Former U.S. Sen. George Mitchell has stepped down from overseeing private settlement talks over a deadly Northern California pipeline explosion after state regulators set up a mediation process criticized by several parties as an unfair, backroom deal. The California Public Utilities Commission appointed the former Senate Majority Leader earlier this month to mediate a settlement aimed at determining how much Pacific Gas & Electric Co. should be fined for the blast. But the cities of San Bruno and San Francisco as well as the commission's own consumer advocacy branch said Mitchell should not preside over talks because the commission had given PG&E advance notice of his hiring.